1. Field of the Invention
The invention relates to stringed musical instruments and, more particularly, to an apparatus internal to the neck of the instrument for adjusting the curvature of the neck.
2. Description of the Prior Art
Stringed musical instruments such as banjos and guitars have been known for many years. In these instruments, tightly wound strings are stretched from the end of a long neck to the base of a sound box to which the neck is fastened. Sound is produced by the vibration of the strings when plucked or when a bow is drawn across them, the sound being amplified by the sound box. Because the strings are stretched tightly from the end of the neck to the base of the sound box, a great deal of tension is placed on the neck; the point on the sound box where the neck is attached acting as a fulcrum. Thus, in response to the strings pulling at the end of the neck, the tendency is for the neck to bow concavely.
Particularly under humid conditions, the neck, often made of wood, will have an even greater tendancy to bow or warp. Thus, strings which have been properly tensioned will gradually loosen because of the neck curving concavely. Further, the distance between the strings and the fingerboard on the neck will increase as the bowing of the neck increases, resulting in poor fingering of the strings. Also, because of the wood construction of the neck and the tension imposed upon it, breakage of the neck is a possibility.
In response to these considerations, several reinforcing and wrap counteracting devices have been introduced. The simplest of these devices is in the form of a metal truss rod having threaded ends. One end engages an anchor, and the opposite end engages a tension nut and an associated washer. The anchor holds the truss rod end in an axially stationary position relative to the instrument's neck. The truss rod is tensioned into a concave configuration relative to the strings and is positioned into an accomodating groove which runs the length of the instrument's neck. The groove includes a cylindrical bore at one end of the neck through which one end of the truss rod protrudes. The tension nut washer, positioned over the end of the protruding truss rod, abuts a shoulder surrounding the bore opening and remains axially stationary thereto. The tension nut is threaded onto the protruding threaded rod end adjacent to the washer. An elongate filler strip, having the same general arcuate shape of the elongate groove and bowed truss rod positioned therein, is glued into the groove, over the truss rod enclosing the truss rod between the strip and the neck of the instrument. Finally a fingerboard is generally glued in place over the filler strip.
Concave bowing of the neck due to the pull of the strings is corrected by rotation of the nut about the threaded truss rod end, causing axial movement of that rod end relative to the washer and instrument neck. Tightening of the nut decreases the effective length of the bowed truss rod between the anchor and washer, causing the rod to pull downwardly on the ends of the neck away from the strings and to force the neck upwardly at the middle of the arcuate filler strip. The net effect of these forces causes the neck to bow convexly relative to the strings. The degree of this convex force is adjustable by tightening or loosening the tension nut.
The concave bow of the neck in this simplest truss rod device generally is limited to the degree of bow imparted to the neck during its construction with the tension nut loose, coupled with the forces exerted by the tightened instrument strings in the completed instrument. This is because no means for increasing the effective truss rod length are provided; the rod can only be shortened. Variations on this simple truss rod theme are exemplified by U.S. Pat. Nos. 4,074,604 to Fender (central portion of truss rod flattened in a plane perpendicular to the bow of the truss rod), and 4,167,133 to Adams (rod replaced by a tensionable strap), the disclosures of which are incorporated herein by reference.
Both concave and convex bowing of a single truss rod to correct neck warp have been advanced in U.S. Pat. No. 3,159,072 to Burns et al, the disclosure of which is incorporated herein by reference. In the Burns device, an anchored toothed wheel-worm gear mechanism alternately lengthens or shortens the effective length of a truss rod between the gear mechanism and a distal rod anchor, the double action providing convex and concave bowing, respectively.
Another device is known for alternately lengthening or shortening effective truss rod length to achieve both concave and convex bowing by a single truss rod device lacking a worm gear mechanism. Simply, a single rod is threaded on both ends, over each of which is threaded an anchor nut. A head is brazed or welded to one end of the rod in order to turn the rod. Thus, with the anchor nuts firmly embedded within and at either end of the instrument's neck, the effective length of the rod between the anchor nuts may be increased or decreased by turning the rod, by its head, thereby providing convex or concave neck bow.
While the foregoing devices ultimately achieve the result of counteracting convex or concave neck warp, several basic problems arise from their use which have not been adequately addressed. The major problem with the foregoing devices is their repair. Because the truss rod is compressed between the filler strip and the instrument's neck, being further overlaid by a fingerboard, dismantling of the neck is necessary to gain access to the rod. Most frequently, the need for repair arises due to the stripping of the threads at the truss rod end. Once the threads are stripped, the tension nut may no longer be tightened. As a consequence the curvature of the neck may no longer be adjusted. Generally, once the threads on a rod are stripped, it is necessary to replace the entire rod. However, removal of the fingerboard and filler strip in order to reach the rod is no easy task, as these are generally firmly glued into position on the instrument's neck. Prying these components loose usually results in their being damaged in some way. Further, even after the rod is exposed, it too must be pried loose from its groove potentially resulting in even further damage to the instrument's neck. Such a consequence would be especially devastating where the instrument being repaired is one of great value or age.
Occasionally, the rod may pull free from its anchor. Repair of this problem is perhaps more easily effected since the rod simply needs to be reattached to its anchor, perhaps by welding or brazing. Although this problem arises infrequently, it still necessitates the removal of those components which cover the anchor in order to reach it.
Another problem relating to the foregoing truss rod devices is the amount of space they take up within the instrument's neck. The neck of the instrument is fragile enough to begin with. Carving out grooves and spaces for rods, anchors and tension nuts removes more wood of the fragile neck than is desirable. One point of the neck especially vulnerable to breakage is the junction between the instrument neck and the peghead. The peghead is a flattened structure contiguous with and at an angle to the end of the instrument's neck. Apertures in the peghead accomodate pegs to which the strings attach. The strings may then be tensioned in order to tune the instrument simply by winding them up onto the pegs. Due to the pulling forces exerted by the strings on the pegboard neck junction, breakage at this point is most common. Hollowing out a portion of the neck in this especially vulnerable region to accomodate either a rod anchor or tension nut results in even greater vulnerability to breakage. Ideally there would be no hollowed out portions at this pegboard neck junction.
In truss rod devices where the rod itself must rotate in order to effectively shorten the rod length, a problem in installation arises. Because the rod must be free to rotate within the neck, the groove accomodating the rod must not bind the rod and, more importantly, glue which attaches the fingerboard, and any filler strip to the neck must not adhere to the rod. While some methods for preventing the rod from being glued are used, such as surrounding the rod with wax paper or periodic rotation of the rod while the glue sets, any failure of these methods necessitates complete dismantling of the neck in order to free up the rod. Also, if the groove is too large for the rod, the rod will vibrate sympathetically with the string vibrations when the instrument is being played.
Although single truss rod devices have been effective in correcting concave warping of the neck, they have been less effective for correcting convex warp because of the need to impose a pretensioned concave bow on the rod within the neck. Further, these devices are difficult to install and repair. In order to satisfy the need for more effective double action truss rod devices, two addition devices have been proposed.
The first of these consists of a rigid sleeve along the length of the neck. Movable core elements within the sleeve cooperate with core elements secured to the sleeve, thereby imparting deflection of the sleeve and neck. Unfortunately, such an apparatus is subject to the same access for repair constraints already discussed. Further, such a device is necessarily complex, and consequently more difficult to manufacture and use, as well as expensive.
A second approach to the problem of effective concave and convex adjustment of neck warp involves using a truss rod apparatus having two rods, an example of which is taught by U.S. Pat. No. 3,416,399 to Baldoni, the disclosure of which is incorporated herein by reference. Here, parallel rods, threaded at each end, are tensioned into a concave bow relative to the strings. The bowed rods are held bowed by a filler strip within an arcuate groove along the length of the neck. Anchors secure each rod at one end of the neck while turnbuckles, anchored at the opposite end of the neck, engage each rod independently in order to increase or decrease the rod's effective length. In so doing, it is possible to counteract concave and convex bowing to the same degree as would be achieved by having two truss rod devices of the type disclosed in the U.S. Pat. No. 3,159,072 to Burns embedded in the neck in parallel relation. Thus, a doubled force to counteract convex and concave warping, as well as some lateral warp correction, can be achieved by using the Baldoni device. However, because wood grains in the instrument neck generally run the length of the neck, lateral warp in unlikely. Further, the repair problem for the Baldoni device now is doubled as two rods having the same drawbacks as all of the previous single truss rod devices are embedded in a single neck.
Yet an additional example of known truss rod devices is a double rod assembly which may be bowed in only one direction. This device consists of two straight parallel rods which are joined together at one end. At their opposite unjoined ends, the rods are attached to an anchor. One rod is fixedly attached to the anchor while the other rod passes through the anchor to thread with a tension nut on the other side. By tightening the tension nut, the effective length of the rod with which it threads is shortened, causing the rod pair to bow. Loosening of the tension nut will return the rod pair to its straight configuration by returning the shortened rod to its original length. This device is installed within the instrument neck in the straight, untensioned condition. Thus, the necessity of an arcuate groove is eliminated. Further, a filler strip is now unnecessary since no structure is needed to maintain the rod in a bowed condition. A fingerboard may be glued directly to the instrument's neck in order to cover the truss rod lying within the straight neck groove. In order to prevent glue from binding the length adjustable rod, the rods are ordinarily covered with a protective cover which allows the rods to slide freely within. Further, because the rods act upon each other in order to bow, there is need for only one anchor. By placing the device within the groove so that the end where the rods are joined together is nearest the pegheadneck junction, very little supportive wood needs to be removed from there. This greatly increases the strength of this region. More importantly, repair is simplified since the entire device within a completed neck can be easily removed from the neck simply by pulling it out from the base of the neck where an opening to the groove is provided to accomodate the anchor. Thus, instead of removing the glued fingerboard and filler strip to gain access to an anchor or tension nut near the fragile pegboard neck junction, the entire apparatus can simply and quickly be removed from the sturdier base of the instrument neck.
The major drawback of this type of apparatus is that it can correct warp in the neck by bowing in only one direction. Presumably, correction of neck warp in an opposite direction could be achieved, but only by removing the apparatus from the base of the neck and turning it over, so that the bow would now be appropriate to counteract that opposite warp. Such a corrective measure is entirely inadequate for day to day use of an instrument. The net result of using known truss rod devices is that concave and convex neck bow can be corrected, but significant shortcomings remain as regards installation, use, and repair.